In a proton exchange membrane (PEM) fuel cell, hydrogen fuel is supplied to a negative electrode (anode) where it catalytically dissociates into protons and electrons according to the oxidation reaction H2→2H++2e−. The protons (H+) pass through a membrane electrolyte to a positive electrode (cathode) while the electrons (e−) are conducted through an external path creating an electric current between the anode and cathode through an external load. At the cathode the protons and electrons recombine in the presence of oxygen to form water according the reduction reaction: O2+4e−+4H+→2H2O. The by-products of the PEM fuel cell reaction are water and heat; the heat requiring that the fuel cell be cooled to maintain an acceptable internal temperature.
A single fuel cell includes a membrane electrode assembly (MEA), —comprising the membrane electrolyte interposed between a pair of electrodes (anode and cathode), —and, adjacent each electrode opposite the membrane electrolyte, an electrically conductive plate that defines the reactant gas flow fields. The gas diffusion plates direct the reactant gases to their respective electrodes, namely, hydrogen fuel to the anode and oxidant to the cathode, and also transport the water byproduct away from the cell.
In order to generate the voltage needed for a given application, many such fuel cells are electrically connected in series and stacked together to form a cell stack assembly (CSA). Sealed to the cell stack assembly are opposed pairs of external manifolds for distributing reactant and exhaust gases (hydrogen and oxidant) to and from the anode and cathode flow fields. The external manifolds may comprise a fuel input/output manifold opposite a fuel turn manifold and an air input/output manifold opposite an air turn manifold, for example. Fuel cell configurations with internal fuel and air manifolds are known.
To absorb the heat generated by the exothermic reaction of fuel and oxygen within the fuel cell, solid cooler plates with interior coolant channels are typically disposed between every two to four fuel cells of the CSA. FIG. 1 represents a typical prior art cooler plate 210 in which a coolant traverses a plurality of coolant fluid flow channels 262 to transfer heat away from the cells. Coolant fluid flow channels 262 communicate with a coolant manifold via inlet and outlet openings 248 and 250 respectively. Fuel inlet and outlet openings (244 and 246 respectively) and oxidant inlet and outlet openings (240 and 242 respectively) communicate with external reactant gas manifolds. Interfacial seals 264 circumscribe the reactant manifold openings (240, 242, 244 and 246) to isolate the central, coolant flow field portion of plate 210 from the reactant streams flowing through the reactant manifolds and CSA flow fields.
Unlike the reactant gas manifolds, which may be internal or external, the coolant manifold for a PEM fuel cell assembly is typically an internal manifold, i.e., it is disposed within the body of the cell stack assembly. Although external coolant manifolds have been used with phosphoric acid fuel cells, as shown in commonly owned U.S. Pat. No. 3,969,145, for example, they are generally not suitable for PEM fuel cells because of the smaller cell area and thinner cell components of the PEM fuel cells compared to phosphoric acid and other types of fuel cells. A disadvantage of an internal manifold, however, is that it reduces the active area ratio and therefore the power density of the cell stack assembly.
For automotive or vehicular applications in which fuel cell power plants must operate in subfreezing temperatures it is highly desirable to use an antifreeze solution as the coolant, which poses another problem for PEM fuel cells. Antifreeze is a poison to the fuel cell catalysts and must not be allowed to come in contact with the cells. The cooler plates and coolant manifold seals must be impervious to the antifreeze solution. This problem was typically addressed in the prior art by implementing both interfacial seals 264 (FIG. 1) and edge seals in plane with, and circumscribing the cells to prevent contamination. Reliable seals, however, are both difficult and expensive to achieve.